Cryptococcosis occurs disproportionately in patients with impaired cell-mediated immunity (CMI), especially those with AIDS. Although development of an effective CMI response is essential in combating this fungus, the role that specific cellular and soluble components of CMI play is incompletely understood. Two virulence factors of C. neoformans have been identified experimentally, its capsule and its ability to produce maelanin, yet it remains unclear how these help the organism elude host defenses. This proposal focuses on the central role of the macrophage (MP) as an effector and affector cell in CMI, in order to define the derangements occurring in the setting of impaired CMI that may allow C. neoformans to elude host defenses. The effector cell role will be probed by exploring why unactivated MP fail to kill C. neoformans, and the requirements to activate MP to kill. MP mediated killing will be determining following treatment with cytokines including gamma interferon and TNF and with the addition of purified non-MP cell populations. MP killing mechanisms will be studied (i) using inhibitors and stimulators of specific MP functions (ii) by determining opsonic and other condition necessary for adherance and internalization of the organisms, (iii) by quantitating generation of microbicidal substances such as oxidants by C. neoformans stimulated MP, (iv) by measuring early cellular events, such as calcium ion transients, which are presumed to relate to the generation of these microbicidal substances and, (v) in cell-free systems containing putative leukocyte fungicidal products. MP antigen presentation will be studied by determining expression of Ia glycoproteins and interleukin-1 during cryptococcal infections. The above studies will compare virulent cryptococcal strain with capsule and melanin deficient strains. Strains will be unopsonized or specifically opsonized with IgG, C3, or both. Completion of these studies should contribute important new information to our basic understanding of cryptococcal infections, and may provide a rational basis for studies in AIDS and other patients with cryptococcal infections.